In many industrial applications which relate to the thermal treatment of a work piece it is desirous to preheat the work piece prior to the latter being further worked on. One specific instance of such preheating is exemplified by a hot stamping or roll coating process.
In both instances, a continuous, or discrete decorative coating material is applied to the surface of a work piece. To achieve a successful transfer operation, however, the work piece, as well as a layer of decorative material to be transferred, are both heated. Concurrently, the transfer layer is applied to the work surface under sufficient pressure to assure a firm transfer.
In a specific example, a thin decorative layer of coating material is applied to a metal, wood, plastic or other type of surface. The coating layer is usually embodied in a composite foil.
A characteristic foil which is frequently used in the above process consists primarily of a thin, plastic carrier film or material, normally polyester or the like. One surface of the carrier film is provided with a heat sensitive release agent.
A protective layer or coating is next applied to the heat sensitive release agent. Thereafter one or more layers of the decorative materials are applied, usually by vapor deposition. These layers can consist of a metal such as aluminum, chromium or the like. They could also comprise solid pigments or multi-colored designs.
The decorative layer or layers are next covered with a thickness of a sizing coating comprising a heat sensitive adhesive. This latter serves to bond the one or more decorative coats to the product surface being treated.
It is understood that many different effects can be produced through the proper combination of colors, metals and the like. In any instance, the decorative layers will be adhered or bonded to the treated work surface whereby to give the latter a desired appearance such as that of wood grain, a decal, or other decorative representation.
In the prior art, the technique of transferring a design layer of material onto a work piece usually involves the concurrent application of heat and pressure. For example, in the art of hot stamping and roll coating, heat and pressure are applied to firmly compress the foil element into engagement with the work piece surface.
Transfer of the decorative material from the foil is achieved by first separating the decorative layer through the expedient of melting a thermally degradable adhesive that holds the layer to a carrier material. The released layer, still under pressure, is now pressed onto the work piece surface to which it adheres.
As presently practiced, transfer of a decorative layer in the manner described has generally embodied the application of heat and pressure to a relatively limited area. Usually, both factors have been applied by way of a heated roller which firmly urges the foil into engagement with the work piece.
The pressure roll or roller is normally provided with a resilient surface to best achieve pressure distribution across the work piece surface. Nonetheless, such an arrangement serves to concentrate the applying effort to the work piece through a substantially line contact with the roller.
As a consequence, the work piece is advanced through the transfer step at a relatively slow rate of speed. The low speed is necessitated in order that the actual dwell period at the transfer point will be sufficient to allow enough heat to flow into the foil and thence to the work piece surface.
This lengthy dwell time is mandated to assure the quality of the transfer and as a result the roller or applicator unit is heated to an excessive temperature. Thus, the overall heat provided to achieve the transfer is far in excess of the actual btu requirement. The excess heat at the roller is thereafter dissipated without achieving any useful function.
It is understandable then, that heat losses present factors which normally increase operating costs. Further, the higher roller temperature represents a factor which accelerates equipment useful life.
In the instance of a hot stamping operation, transfer of the decorative layer is achieved by the application of heat and pressure through a stamping pad or element. The latter is essentially heated to a temperature sufficient to cause separation as noted herein, of the decorative layer from the carrier material.
Thus, the stamping head or die is heated to a sufficient temperature to achieve the necessary melting and transfer steps. Practically, the stamping pressure and the heat are applied while the work piece is held stationary beneath the head, the foil being compressed therebetween. Although the transfer operation can be achieved, it is clear that the expenditure of heat as well as dwell time, are excessive.
Toward overcoming the above noted problems, the present apparatus provides a means for achieving a more judicious application of heat to a work piece during a roll coating or hot stamping operation. Physically, the apparatus provides means for increasing the actual dwell time of a heating element against the surface of a work piece. Concurrently, it provides means for more rapidly advancing a work piece to realize an overall increased production rate.
The dwell time of the heating element on the work piece as herein disclosed, is achieved through use of a heated, continuous thermal belt which contacts the work piece at a preheat station. The transfer station is thus extended substantially from a limited or line contact area, to an elongated contact surface moving in the direction of the work piece travel.
Means is therefore provided for accurately and judiciously metering heat to a work piece, as well as to a transfer foil. Said heating means is embodied in a continuous thermal belt which is sequentially guided into heat exchange relation with a heat source, and thence with the transfer member.
The continuous thermal belt is carried on a belt guide system comprised of at least two, and preferably a plurality of driven rollers. The latter are journalled, and so arranged to maintain the belt in a condition of tension throughout a closed circuit. A heat source is positioned sufficiently near to a portion of the belt's closed circuit to permit the belt to be controllably heated as it passes close to or contiguous with the heat source.
To maintain a desired heat level at the transfer station, one or more of the driven belt guide rollers can be provided with a supplemental heating source. The latter can conveniently embody a heater element and either electricity, steam or other form of acceptable energy source compatible with the roller shape.
To best assure the desired decorative layer transfer, the work piece is preferably preheated by the moving thermal belt through physical contact with the belt. This heat exchange is initiated prior to contact between the work piece and the foil member. Thus, the work piece which is normally of a much greater thickness than is the foil, can conduct either the major part of the heat required by the latter, or at least a portion of the heat requirement.
Stated otherwise, what applicant has provided is means for metering an accurately determined amount of heat to a work piece and to a transfer foil, whereby to best accomplish a transfer of the decorative layer from the latter onto the work piece. Further, by controllably regulating the dwell time between transfer foil and work piece, it is possible to greatly decrease the overall operation time, and to decrease the per unit cost.
It is an object of the invention therefore to provide an improved apparatus and method for achieving a more rapid and efficient transfer of a decorative layer to the receiving surface of a work piece. A further object is to provide an apparatus capable of more effectively delivering a controlled flow of heat to a work piece. A still further object is to provide a novel heat transfer apparatus capable of more efficiently regulating the heat flow required to achieve a heat transfer operation onto a work piece.